We are studying factors affecting transmission through a spinal reflex circuit in the mammalian central nervous system. The reflex chosen for investigation is the plantar cushion (PC) reflex, a cutaneous reflex in the cat. The basic neural circuit for the PC reflex appears to consist of not more than two serially connected dorsal horn ceels, interposed between the cutaneous afferents from the plantar cushion and motoneourons innervating the intrinsic plantar muscles. We intend to stain representatiies of the various neuronal elements of the PC reflex circcit, ussng the intracellular horseradish peroxidas technique, to clarify anatomical details of this circuit. Intracellular recordings from first-order dorsal horn cells responding to PC stimulation will be made during iterated stimulation, to investigate whether the changes in reflex transmission during iterated stimulation that we have previously described are primarily due to pre- or postsynaptic mechanisms. We also intend to continue our studies of classical conditioning of the PC reflex in acute preparations, to determine whether the conditioning-like changes we have observed may validly be viewed as vaidly be viewed as a form of learning, or rather simply reflect generalized changes in reflex excitability. If specific conditioning of the PC reflex can be demonstrated convincingly, we will use the PC reflex to study neural mechanisms related to changes in transmission during conditioning.